In recent years, large liquid crystal cells have been used in flat panel displays. The liquid crystal cells are normally constructed by two glass plates joined together with a layer of a liquid crystal material sandwiched inbetween. The glass substrates have conductive films coated thereon with at least one of the substrates being transparent. The substrates are connected to a source of power to change the orientation of the liquid crystal material. A possible source of power is a thin film transistor that is used to separately address areas of the liquid crystal cells at very fast rates. The TFT driven liquid crystal cells can be advantageously used in active matrix displays such as for television and computer monitors.
As the requirements for resolution of liquid crystal monitors increase, it becomes desirable to address a large number of separate areas of a liquid crystal cell, called pixels. For instance, in a modem display panel, more than 3,000,000 pixels may be present. At least the same number of transistors must therefore be formed on the glass plates so that each pixel can be separately addressed and left in the switched state while other pixels are addressed.
Thin film transistors are frequently made with either a polysilicon material or an amorphous silicon material. For TFT structures that are made of amorphous silicon material, a common structure is the inverted staggered type which can be back channel etched or tri-layered. The performance of a TFT and its manufacturing yield or throughput depend on the structure of the transistor. For instance, the inverted staggered back channel etched TFT can be fabricated with a number of six masks, whereas other types of inverted staggered TFT require a minimum number of nine masks.
A second type of TFT is made by using a polysilicon material. Polysilicon is more frequently used for displays that are designed in a smaller size, for instance, up to three inch diagonal for a projection device. At such a small size, it is economical to fabricate the display device on a quartz substrate. Unfortunately, large area display devices cannot be normally made on quartz substrates. The desirable high performance of polysilicon can therefore be realized only if a low temperature process can be developed to enable the use of non-quartz substrates. For instance, in a more recently developed process, large area polysilicon TFT can be manufactured at processing temperatures of less than 600.degree. C. In the process, self-aligned transistors are made by depositing polysilicon and gate oxide followed by source/drain regions which are self-aligned to the gate electrode. The device is then completed with a thick oxide layer, an ITO layer and aluminum contacts.
Polysilicon TFTs have the advantage of a high charge current and the drawback of a high leakage current. It is difficult to maintain the voltage in a pixel until the next charge in a polysilicon TFT due to its high leakage current. Polysilicon also allows the formation of CMOS devices, which cannot be formed by amorphous silicon. For the fabrication of larger displays, a higher mobility may be achieved by reducing the trap density around the grain boundaries in a hydrogenation process.
FIG. 1 shows an enlarged, cross-sectional view of a conventional amorphous silicon TFT structure. A typical active matrix LCD panel 10 is shown which consists of an array of pixels 12. Each pixel 12 is activated by addressing simultaneously a designated drive line 14, or data line, and gate line 16. A drive element 18, which is a thin film transistor, is associated with each pixel. The drive lines 14, gate lines 16, pixels 12 and pixel drive elements 18 are deposited on a clear glass substrate by a photolithographic process. In a device with high pixel densities, the close proximity of the gate lines and drive lines, and the complexity of forming the drive elements for the pixels, i.e., the thin film transistors, defects are frequently formed in the structure during the fabrication process. For instance, in an LCD array of 640 by 480 pixel elements, a few structural defects in the circuits of the pixels are frequently observed during reliability tests.
Traditional test methods for high density LCD panels require physical contacts to be made such that connection to and testing of each individual row/column intersection within a panel array can be made. In more advanced testing methods, advanced probing technology has been used for establishing reliable contacts among the densely populated pixel elements. For instance, one of such test can be conducted in a commercially available equipment as shown in FIGS. 2A and 2B. An LCD panel 10 is provided with a plurality of conductive leads 20 in the horizontal direction and a plurality of conductive leads 22 in the vertical direction. The conductive leads 20, 22 are provided in the form of tape automated bonding (TAB) leads. The plurality of conductive leads 22 are connected to the gate lines of the LCD panel 10, while the conductive leads 20 are connected to the drive lines of the LCD panel 10. A conventional test fixture 30 is provided which includes a vertical side frame 32 and a horizontal side frame 34. On the vertical side frame 32, a multiplicity of conductive elements (not shown) are provided on a printed circuit board 36, or a Y card, On the horizontal side frame 34, a multiplicity of conductive elements (not shown) are provided on a printed circuit board 38, or the X card. The printed circuit boards 36, 38 are mounted to the side frames 32, 34 respectively by locating pins 40.
The printed circuit boards 36, 38 are constructed by conventional techniques such as aluminum traces formed on an insulating board such as epoxy. The horizontal side frame 34 and the vertical side frame 32 are permanently mounted on a base frame 28 which also provides guiding blocks 26 in the test fixture 30 shown in FIG. 2A. The configuration of the fixture 30 is fixed such that it can only be used for an LCD panel that has the specific dimensions, for instance, a 3-inch.times.4-inch LCD panel. FIG. 2B is a perspective view showing the LCD panel 10 positioned on the test fixture 30. In the position shown in FIG. 2B the conductive leads on the PCB boards 36, 38 establish electrical communication with the conductive elements on TAB 22, 20 of the LCD panel 10. A test of the LCD panel 10 can thus be conducted. When a different sized LCD panel is to be tested, a different test fixture must be provided to accomplish such task. The test fixture shown in FIGS. 2A and 2B is expensive to build due to the high accuracy required and the expensive material used for achieving dimensional stability at different ambient temperatures.
By utilizing the conventional test fixture 30 shown in FIG. 2A, a plurality of conventional tests can be conducted. These tests are shown in FIGS. 3A.about.3E. For instance, FIG. 3A illustrates a simplified, equivalent circuit for a test that can be conducted with a single gate line 42 and a single drive line 44. In this configuration, all the gate lines 46 and all the drive lines 48 are shorted together, respectively to form a single gate line 42 and a single drive line 44. In this connecting point contact design of 1G1D, a test can be carried out at low cost for a quick detection of any open circuits since only one input signal is required for the gate line and for the drive line, respectively. In this configuration, all the LCD panel ITO pads are connected to the printed circuit board on the test fixture, while the conductor layout of the gate and data PCB are shorted, respectively. This is a low cost test method where the PCB can be fabricated at low cost and reused.
Contrary to the 1D1D test configuration, a full contact probing method is shown in FIG. 3B. In this configuration, 6 individual gate lines 46 and 6 individual drive lines 48 are utilized to enable a variety of tests based on different configurations such as 1G1D, 1G2D, 2G2D, 2G3D, and full contact probing. The pressure conductive rubber (PCR) utilized in the test fixture need not be severed and need not be changed. The drawback of the full contact probing configuration is that the PCB is only adapted to accept a single LCD panel, i.e., its ITO pattern and pitch. When different ITO pattern and pitch are involved, a new design PCB must be utilized. Other test configurations such as 1G2D, 2G2D, and 2G3D are shown in FIGS. 3C, 3D and 3E, respectively.
It is therefore an object of the present invention to provide an LCD panel power-up test fixture that does not have the drawbacks or shortcomings of the conventional fixtures.
It is another object of the present invention to provide an LCD panel power-up test fixture that is adjustable such that panels of different sizes can be tested on the same fixture.
It is a further object of the present invention to provide an adjustable LCD panel power-up test fixture by providing a base plate and mounting at least two side frames on the base plate wherein the side frames may be slidingly adjusted to accommodate panels of different sizes.
It is another further object of the present invention to provide an adjustable LCD panel power-up test fixture wherein at least three side frames are utilized which slidingly engaging each other for adjusting to a specific LCD panel size.
It is still another object of the present invention to provide an adjustable LCD panel power-up test fixture wherein printed circuit boards are provided on the side frames for engaging TAB leads on the LCD panel for establishing electrical communication thereinbetween.
It is yet another object of the present invention to provide an adjustable LCD panel power-up test fixture wherein four side frames each having an L-shape slidingly engaging each other for accommodating a specific LCD panel size.
It is still another further object of the present invention to provide an adjustable fixture for LCD panel power-up test by providing four side frames slidingly engaging each other and each is provided with conductive elements on a top surface of the frame.
It is yet another further object of the present invention to provide an adjustable fixture for LCD panel power-up test which is constructed by four side frames each having a top surface provided with printed circuit boards and conductive elements on the boards such that LCD panels of any size may be tested.